1. Field of the Invention
The present invention relates to a fixing apparatus designed for use in an image forming apparatuses such as copiers or printers and configured to fix developer images on paper sheets.
2. Description of the Related Art
Any image forming apparatus utilizing digital technology, such as an electronic copier, comprises a document table and a photoelectric transducer such as a CCD (Charge Coupled Device). An original document is placed on the document table so that it may be copied. Light is applied to, and reflected from, the original document. The light reflected is guided to the photoelectric transducer.
The CCD generates an image signal that represents the image printed on the original document. The image signal is supplied to the laser provided in the apparatus. Driven by the signal, the laser emits a laser beam. The laser beam is applied to the photosensitive drum incorporated in the apparatus, forming an electrostatic latent image on the circumferential surface of the drum. Developer is applied to the drum, converting the latent image to a visible image known as “toner image.” A paper sheet is fed to the drum as the drum is rotated. The toner image is transferred to the paper sheet. The paper sheet, now having the toner image on it, is fed to the fixing apparatus provided in the image-forming apparatus.
The fixing apparatus comprises a heating roller and a pressing roller. The pressing roller contacts the heating roller. It rotates together with the heating roller, applying a pressure onto the heating roller. The paper sheet is fed forward through the nip between the heating roller and the pressing roller. As the sheet is fed so, the toner image is fixed on the paper sheet by virtue of the heat generated by the heating roller.
The heat of the heating roller is generated induction heating. The induction heating is performed by a resonant circuit that comprises a coil contained in the heating roller and a capacitor connected to the coil. The resonant circuit is excited at a frequency, passing a high-frequency current through the coil. The coil generates a high-frequency magnetic field, which induces an eddy current. The eddy current brings forth Joule heat, which heats the heating roller.
The resonant circuit has a specific resonance frequency that is determined by the inductance of the coil and the electrostatic capacitance of the capacitor. The resonance frequency may be, for example, 2 MHz. In this case, the output power of the resonant circuit may reach, for example, 1500 W.
Any fixing apparatus that comprises a resonant circuit of such a high resonance frequency and such a large output power is undesirable in view of EMI (Electromagnetic Interference). That is, the high-frequency magnetic field emanating from the coil provided in the resonant circuit adversely influences the other components and devices incorporated in the fixing apparatus.